Cialis is a medication used to treat erectile dysfunction. It belongs to a class of medications called phosphodiesterase type 5 inhibitors. These medications work by relaxing the muscles in the penis and increasing blood flow to the area, which helps to achieve and maintain an erection, read here. Cialis can be taken with or without food. It should not be taken more than once a day. Cialis should be taken at least 30 minutes before sexual activity. Possible side effects include headache, flushing, upset stomach, and muscle pain.